title: A 7-deoxyloganetic acid glucosyltransferase contributes a key step in secologanin biosynthesis in Madagascar periwinkle.
abstract: Iridoids form a broad and versatile class of biologically active molecules found in thousands of plant species. In addition to the many hundreds of iridoids occurring in plants, some iridoids, such as secologanin, serve as key building blocks in the biosynthesis of thousands of monoterpene indole alkaloids (MIAs) and many quinoline alkaloids. This study describes the molecular cloning and functional characterization of three iridoid glucosyltransfeases (UDP-sugar glycosyltransferase6 [UGT6], UGT7, and UGT8) from Madagascar periwinkle (Catharanthus roseus) with remarkably different catalytic efficiencies. Biochemical analyses reveal that UGT8 possessed a high catalytic efficiency toward its exclusive iridoid substrate, 7-deoxyloganetic acid, making it better suited for the biosynthesis of iridoids in periwinkle than the other two iridoid glucosyltransfeases. The role of UGT8 in the fourth to last step in secologanin biosynthesis was confirmed by virus-induced gene silencing in periwinkle plants, which reduced expression of this gene and resulted in a large decline in secologanin and MIA accumulation within silenced plants. Localization studies of UGT8 using a carborundum abrasion method for RNA extraction show that its expression occurs preferentially within periwinkle leaves rather than in epidermal cells, and in situ hybridization studies confirm that UGT8 is preferentially expressed in internal phloem associated parenchyma cells of periwinkle species.
authors.affiliation: Department of Pharmacognosy, Graduate School of Pharmaceutical Sciences, Nagoya City University, Mizuho-ku, Nagoya 467-863, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Asada
authors.forename: Vonny
authors.initials: V
authors.lastname: Salim
authors.forename: Sayaka
authors.initials: S
authors.lastname: Masada-Atsumi
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Edmunds
authors.forename: Mai
authors.initials: M
authors.lastname: Nagatoshi
authors.forename: Kazuyoshi
authors.initials: K
authors.lastname: Terasaka
authors.forename: Hajime
authors.initials: H
authors.lastname: Mizukami
authors.forename: Vincenzo
authors.initials: V
authors.lastname: De Luca
mesh_terms: D030008:Catharanthus / Q000201:enzymology* / Q000235:genetics; D003001:Cloning, Molecular; D020868:Gene Silencing; D005964:Glucosyltransferases / Q000235:genetics / Q000378:metabolism*; D057888:Iridoid Glucosides / Q000378:metabolism*; D008969:Molecular Sequence Data; D052585:Phloem / Q000166:cytology / Q000201:enzymology; D010802:Phylogeny; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D046948:Secologanin Tryptamine Alkaloids / Q000378:metabolism
chemical_list: D057888:Iridoid Glucosides; D010940:Plant Proteins; D046948:Secologanin Tryptamine Alkaloids; C021086:secologanin; D005964:Glucosyltransferases